Attaches N-acetylneuraminic acid in α-2,6-linkage to N-acetyl-galactosamine only when present in the structure of α-N-acetyl-neuraminyl-(2→3)-β-galactosyl-(1→3)-N-acetylgalactosaminyl-R, where R may be protein or p-nitrophenol. Not identical with EC 2.4.99.3 α-N-acetylgalactosaminide α-2,6-sialyltransferase.